Thermoformable Copolyester Laminate

ABSTRACT

A laminate includes the following substantially coextensive layers in the following order: (a) a non-sealable, self-supporting, thermoformable copolyester film layer having a first surface and a second surface, the second surface constituting an outermost, exposed surface of the laminate; (b) a laminating adhesive layer on the first surface of the thermoformable copolyester film layer; and (c) a self-supporting, thermoformable structural film layer having a first surface and a second surface, the first surface contacting the laminating adhesive layer. Polyethylene terephthalate constitutes at least 80% by weight of the self-supporting thermoformable copolyester film layer; the thermoformable structural film layer includes a polymer selected from the group consisting of polyamides, polypropylene, polyethylene, polyethylene terephthalate, ionomers, ethylene acrylic acid copolymers, ethylene vinyl acetate copolymers, polystyrene, ethylene vinyl alcohol copolymers and polyvinylidene chloride; the thermoformable copolyester film layer, the structural film layer and the laminate each shrink less than 5% in length and width upon exposure to boiling water for five seconds; and the laminate is thermoformable and its chloroform-soluble extractives meet the requirements of paragraph h(1) of 21 CFR §177.1630 as defined herein.

BACKGROUND OF THE INVENTION

Articles such as food items and more specifically meat, poultry andseafood products are often packaged in thermoplastic films or laminatesto protect the product from exterior abuse and environmentalcontamination, and to provide a convenient and durable package fordistribution of the product and display in a display case or other pointof sale. Packages made from films that are formed, filled with productand sealed exist in many shapes and forms for many applications, and arecommonly used for such packaging. When dealing with meat products inparticular, it is usually desirable to provide a film having good oxygenbarrier characteristics, in order to reduce the passage of oxygenthrough the film so that detrimental effects on freshness, color, andother properties of the meat product are minimized.

In many cases it is desirable to provide a package that can be used tocook the contained foodstuff without removing the packaging material (a“cook-in” package). It is also desirable that such packages be suitablefor heating or cooking in conventional, convection or microwave ovens(“dual-ovenable”) or for heating in boiling water. It is particularlyconvenient to be able to purchase a fresh, partially, or fully cooked orvalue-added foodstuff in a package at the retail level and merely insertthe package directly into a conventional or microwave oven and cook andreheat the foodstuff.

Thermoforming methods such as vacuum forming or plug-assist vacuumforming are often used to produce suitable packages. In general terms,thermoforming involves heating a thermoplastic film or laminate andforming it into a shape suitable for containing the product, which issubsequently inserted. The film or laminate is sometimes referred to asa “forming web”, and the package is closed with a “capping web” filmthat is adhered to the thermoformed package.

Coextruded films comprising polyolefins such as polyethylene aresometimes useful in producing thermoformed packages. Laminate films havealso been employed. For example, U.S. Pat. No. 4,940,634 disclosesbiaxially oriented thermoplastic composite films comprising polyolefins,suitable for use as forming webs in thermoforming operations. It iscommon practice to join the capping web to the thermoformed package byheat-sealing the two together. Each films accordingly has aheat-sealable layer on its surface, or consists entirely of aheat-sealable polymer. The two films are pressed together with heatingto effect the bond.

However, films comprising polyolefins generally do not possess hightemperature heat resistance, which limits their application to microwavecooking or reheating. On the other hand, polyethylene terephthalate(PET) films have high heat resistance, making them suitable for cook-inuses, but PET itself is not heat-sealable except at exceptionally hightemperatures. If a heat-seal layer is provided on both PET surfaces, anacceptable seal can be obtained, but this adds time and expense to thefilm manufacturing process. However, if ones attempts to heat-seal a PETfilm having a heat-seal adhesive layer to a typical polyester filmhaving no heat-seal adhesive layer, the bond strength is unacceptablyweak for most applications. The need to provide a heat-seal layer onboth film surfaces adds time and expense to the film manufacturingprocess. Thus, it would be advantageous to provide a thermoformablepolyester film that does not bear a separate heat-seal layer, but thatis nonetheless capable of being effectively heat-sealed to a polyestercapping web that does.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a laminate including the followingsubstantially coextensive layers in the following order:

(a) a non-sealable , self-supporting, thermoformable copolyester filmlayer having a first surface and a second surface, the second surfaceconstituting an outermost, exposed surface of the laminate;

(b) a laminating adhesive layer on the first surface of thethermoformable copolyester film layer; and

(c) a self-supporting, thermoformable structural film layer having afirst surface and a second surface, the first surface contacting thelaminating adhesive layer.

Polyethylene terephthalate constitutes at least 80% by weight of theself-supporting thermoformable copolyester film layer;

the thermoformable structural film layer includes a polymer selectedfrom the group consisting of polyamides, polypropylene, polyethylene,polyethylene terephthalate, ionomers, ethylene acrylic acid copolymers,ethylene vinyl acetate copolymers, polystyrene, ethylene vinyl alcoholcopolymers and polyvinylidene chloride;

the thermoformable copolyester film layer, the structural film layer andthe laminate each shrink less than 5% in length and width upon exposureto boiling water for five seconds; and

the laminate is thermoformable and its chloroform-soluble extractivesmeet the requirements of paragraph h(1) of 21 CFR §177.1630 as definedherein.

In another aspect, the invention provides a package including a laminateas described above, wherein the laminate is in the shape of athermoformed receptacle having a pocket with a flange and an opening,the second surface of the copolyester film layer forming an innersurface of the pocket and a mating surface of the flange, and aheat-sealable film. The opening of the receptacle is closed by theheat-sealable film, which includes a self-supporting polymeric filmhaving on a surface thereof a substantially coextensive heat-seal layer.The heat-seal layer of the heat-sealable film is in contact with andbonded to the mating surface of the flange of the thermoformedreceptacle, and chloroform-soluble extractives of the heat-sealable filmmeet the requirements of paragraph h(1) of 21 CFR §177.1630 as definedherein.

In yet another aspect, the invention provides a method of packaging afoodstuff, including placing the foodstuff in the pocket of the laminateas described above and subsequently sealing the opening by heat-sealinga film to the flange.

DETAILED DESCRIPTION OF THE INVENTION

Laminates according to the invention include a non-sealablethermoformable copolyester film layer on a surface of the laminate,bonded by a laminating adhesive to a thermoformable structural filmlayer. As used herein, the term “non-sealable” means that attempts tobond two like pieces of the material together by heat-sealing using thetest method specified in the Examples fail to produce a bond strength ofat least 200 g/25 mm. While the copolyester face of such a laminate isnot itself heat-sealable, it is receptive to heat-sealing, meaning thatanother film having a heat-sealable surface layer can form a strong bondto the copolyester face of the inventive film under heat-sealingconditions.

Laminates according to the invention meet the requirements of paragraphh(1) of 21 CFR §177.1630 dated Apr. 1, 2003, incorporated herein asAppendix A, and are dual-ovenable. Laminates and multilayer filmsdescribed herein will be understood to have all of the layers mutuallycoextensive, unless the context makes it clear otherwise. Each of thelayers of the laminate will now be described in detail, followed by adescription of the structure and use of suitable capping webs that maybe used to seal thermoformed packages made from the laminates.

Thermoformable Copolyester Film Layer

Thermoforming comprises heating a film to a temperature (T₁) above theglass transition temperature (T_(g)) of the material but below thecrystalline melting temperature (T_(m)) of the material (if it has acrystalline component), and then applying a deforming force to thematerial while it is in its softened, rubbery, solid state. The film isthen cooled to a temperature below its glass transition temperature, andit must retain the deformation that was introduced while it was in thesoftened rubbery state.

In addition, the elongation (strain) at break (ETB) should be greaterthan the strains experienced during the thermoforming operation, and thetensile strength at maximum elongation (UTS) should be greater than theyield stress.

Thermoformability requires that the deformed film retains the deformedshape, once cooled. Accordingly, an important characteristic of athermoformable film is relaxation of induced stress at the processingtemperature after stretching the film to the desired strain. Thischaracteristic is usually expressed as a percentage of stress retainedafter a defined time period (in seconds), or as the time required torelax stress by a defined percentage, and in a thermoformable film thevalues of these parameters should be as low as possible, as is wellknown in the art (see for instance “Viscoelastic Properties ofPolymers”; John D. Ferry, page 8 et seq., 3rd Ed, Wiley, NY; ISBN0-471-04894-1; and “Mechanical Properties of Solid Polymers”, I. M.Ward, 2^(nd) Ed., John Wiley)).

For purposes of this invention, a film or laminate will be consideredthermoformable if the elongation (strain) at break when measured at 130°C. is greater than 220% in both machine (MD) and transverse (TD)directions.

The thermoformable copolyester film layer is partially amorphous, withthe result that shrinkage of the layer (prior to any thermoforming) istypically very low upon heating. Typically the shrinkage is less than5%, more typically less than 3%, in both the machine (MD) and transverse(TD) directions (i.e., in both length and width). The ability of thelayer to be thermoformed without substantial shrinkage is important,because excessive shrinkage may lead to forming difficulties such asloss of ability to maintain grip on the film during the forming step.

The crystallinity percentage indirectly gives a measure of the amorphouscontent of the polymer, with lower crystallinity meaning higheramorphous content, which in turn may give an indication of the abilityof a film to thermoform. In some embodiments, the thermoformablecopolyester has a crystallinity percentage below about 50%, morepreferably below about 45%, more preferably in the range from 5 to about42%, more preferably in the range from 3 to about 40%.

The thermoformable copolyester film layer is a self-supporting film orsheet, by which is meant a film or sheet capable of independentexistence in the absence of a supporting base. A film-formingthermoplastic copolyester resin constitutes the major component of thethermoformable copolyester film layer, and makes up at least 50%,preferably at least 65%, preferably at least 80%, preferably at least90%, and preferably at least 95% by weight of the total weight of thelayer. Typically, one or more fillers may make up the balance of thelayer. In some cases, the layer consists of the film-formingthermoplastic copolyester resin.

The synthetic linear copolyesters useful for the thermoformablecopolyester film layer may be obtained by condensing the dicarboxylicacids or their lower alkyl diesters, e.g. terephthalic acid (TA),isophthalic acid, phthalic acid, 2,5-, 2,6- or2,7-naphthalenedicarboxylic acid, succinic acid, sebacic acid, adipicacid, azelaic acid, 4,4′-diphenyldicarboxylic acid,hexahydro-terephthalic acid or 1,2-bis-p-carboxyphenoxyethane(optionally with a monocarboxylic acid, such as pivalic acid) with oneor more glycols, particularly an aliphatic or cycloaliphatic glycol,e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol,1,4-cyclohexanedimethanol and diethylene glycol. In general, suitableglycols are of low molecular weight (i.e. below about 250), and in someembodiments it is desirable to avoid the use of glycols having anaverage molecular weight of over 250, such as poly(alkylene oxide)glycols. Aliphatic glycols are preferred, particularly ethylene glycol.In some embodiments, diethylene glycol constitutes at least 1.5 mol % ofthe total diol, or at least 2 mol %, with the balance being ethyleneglycol. In some embodiments, at most 5 mol % is diethylene glycol, or atmost 3.5%.

The copolyester contains at least one aromatic dicarboxylic acid,preferably selected from the aromatic dicarboxylic acids noted above,and preferably said at least one aromatic dicarboxylic acid is TA. Inone embodiment, the copolyester contains only one aromatic dicarboxylicacid, which is preferably TA. In some embodiments, it is desirable toavoid the use of dicarboxylic acid monomers that are sulphonated, i.e.dicarboxylic acids containing a sulphonic acid group or salt thereof(i.e. dicarboxylic acids which contain an —SO₃X moiety where X is H oran alkali metal, such as sodium sulfo-isophthalate).

The copolyester further contains at least one (typically only one)saturated aliphatic dicarboxylic acid of the general formulaC_(n)H_(2n)(COOH)₂ wherein n is 2 to 8, such as azelaic acid. In oneembodiment, the dicarboxylic acid fraction of the copolyester of thethermoformable copolyester film layer consists of at least one (andpreferably only one) aromatic dicarboxylic acid, as defined above, andat least one (and preferably only one) aliphatic dicarboxylic acid, asdefined above.

Preferably, the copolyester comprises at least 90 mol % relative to thetotal diacid fraction of the copolyester of an aromatic dicarboxylicacid, with the remainder being an aliphatic dicarboxylic acid. Moretypically, the amount is at least 92 mol %. Typically, at most 97 mol %of the total diacid is aromatic, more typically at most 95 mol %.

The thermoformable copolyester film layer may contain recycle materialup to a level of 50% by weight of the layer, and preferably at least10%, preferably at least 25%, and more preferably at least 40% by weightof the layer. By “recycle material”, we mean waste material consistingof the composite film of the present invention, and such waste materialmay be derived from edge-trimming (typically the edge portions of thefilm which are held by the stenter clips during film manufacture), fromexcess film left over after the film has been slit along itslongitudinal dimension, from start-up film (i.e. the film produced atthe start of a manufacturing run), or from film that has been failed forother reasons, as is well-known in the art. It is surprising thatrecycle material may be used in the thermoformable copolyester filmlayer in such high proportions given that it contains the wax from theheat-sealable layer without causing problems in the film making process.

The thermoformable copolyester film layer may comprise one or morediscrete coextruded sublayers of the above film-forming materials. Thepolymeric materials of the respective sublayers may be the same ordifferent. For instance, the thermoformable copolyester film layer maycomprise one, two, three, four or five or more sublayers, and typicalmulti-layer structures may be of the AB, ABA, ABC, ABAB, ABABA or ABCBAtype. Preferably, the thermoformable copolyester film layer ismonolithic and comprises only one layer, i.e., multiple coextrudedcopolyester layers are not present. The thermoformable copolyester layertypically has a thickness in a range from 0.5 mil to 4 mil, depending onthe desired end structure. It can be unoriented, but typically it isbiaxially oriented.

Laminates according to the invention are capable of forming a heat-sealbond (on the thermoformable copolyester side) to a capping web with abond strength typically of at least 250 g/25 mm, and more typically atleast 400 g/25 mm, when measured as described in the Examples, usingMYLAR® OL13 film (DuPont Teijin Films, Richmond, Va.) as the cappingweb. The heat-seal bond strength will typically be at most 2500 g/25 mm,and more typically at most 1000 g/25 mm. Packages according to theinvention may use capping webs other than MYLAR® OL13 film, but theranges of heat-seal bond strengths between the laminate and the cappingweb will still fall within the ranges described above. As noted earlier,the thermoformable copolyester film layer of laminates according to theinvention is itself non-sealable, meaning that when bonded to itself theheat-seal strength is less than 200 g/25 mm when measured as describedin the Examples.

Laminating Adhesive Layer

The thermoformable copolyester layer is laminated to the structural filmlayer by use of a laminating adhesive, which may for example be apolyester urethane. The adhesive may typically be applied as a solution.

Solvent-based adhesives can be applied to one side (or two sides) of thebase sheet by any means known to those of skill in the art. For example,the film may be coated by roller (e.g. doctor roll) coating, spraycoating, gravure coating, or slot coating, preferably roller or gravurecoating using a solution coating process.

For example, a two-part polyester urethane adhesive can be applied tothe thermoformable PET via a gravure cylinder to serve as the laminatingadhesive. The laminating adhesive is applied across the web fromsolution. Removal of any solvent may require the application of heat.The two film layers are then laminated using standard laminatingconditions.

The laminating adhesive may also be “solventless.” Solventlesslaminating adhesives are well known in the art and illustrativelyinclude waterborne acrylic emulsions, polyurethane dispersions and oneand two part polyurethane systems with 100% solids. Waterborne systemsrequire dryers after adhesive application at elevated temperatures toeliminate the water before combining with another substrate. On theother hand, polyurethane systems with 100% solids rely on a chemicalreaction for curing and little or no heat is required. In someapplications it is preferred that the laminating adhesive beelastomeric, with exemplary embodiments being polyurethanes.

The laminating adhesive can be applied either to the thermoformablecopolyester film layer or the structural film layer, or both. One orboth of these films may also be surface treated, such as by corona. Insome cases both surfaces may be corona treated prior to application ofan adhesive in order to promote better bonding between the film surfacesin contact with the applied adhesive. The laminating adhesive can beapplied by well known coating techniques such as metering a lowviscosity adhesive onto a multiple application roll system configurationthat applies the adhesive to a first web or substrate. The first web isthen mated to a second web or substrate by use of a heated nip roll.

Thermosetting compositions may also be used for the laminating adhesive.One such adhesive consists of equal parts by volume of MOR-FREE™ 225polyester polyol and MOR-FREE™ C-33 isocyanate, both available from Rohmand Haas of Spring House, Pa. Other examples include solvent adhesivessuch as ADCOTE™ 812 and ADCOTE™ 811B or a mixture of ADCOTE™ 250HV andCoreactant 86, all available from Rohm and Haas of Spring House, Pa. Thelaminating layer typically has a thickness in a range from 0.5 mil to 10mil including the thickness of the adhesive layer.

Thermoformable Structural Film Layer

The thermoformable copolyester film layer is adhesively laminated to athermoformable structural film layer to enhance performance of theoverall structure, depending on the packaging need and intendedcondition of use. Examples of polymeric films useful for the structuralfilm layer are polyamide (for example, nylon), polypropylene,polyethylene, ionomer, ethylene acrylic acid copolymer, ethylene vinylacetate copolymer, polyethylene terephthalate, polystyrene, ethylenevinyl alcohol and polyvinylidene chloride. One exemplary ionomer is soldby E.I. du Pont de Nemours and Company, Wilmington, Del. (DuPont) underthe trade name SURLYN®.

In some embodiments, the layer may be an abuse layer comprising orientedpolyamide (nylon). This layer is preferably unaffected by the sealingtemperatures used to make the package. The thickness of this layer cancontrol the stiffness of the package, and may be in a range from about10 to about 250 μm (0.4 to 10 mils), typically in a range from 50 to 200μm (2 to 8 mils). This layer may be provided with graphic elements suchas printing and embossing to provide information for the consumer and/ora pleasing appearance to the package. Preferably this layer is reverseprintable. The particular film used will in part depend upon the end useof the package. For example, packages containing bones or other hardprojections will require thicker film laminate structures. The thicknessof the laminate structure will also depend on the depth of the drawdesired during thermoforming. A preferred material for the strengthlayers is a polyamide such as biaxially oriented nylon from about 0.5mil to about 5 mils in thickness. Nylon used as an outer layer can befrom about 1 to 5 mils thick. Nylon used as an inner layer incombination with another layer can be from about 0.5 to 10 mils thick.

Polyamides suitable for use in making the structural film layer includealiphatic polyamides, amorphous polyamides, or a mixture thereof.“Aliphatic polyamides” as the term is used herein can refer to aliphaticpolyamides, aliphatic copolyamides, and blends or mixtures of these.Preferred aliphatic polyamides for use in the invention are nylon 6,6;nylon 6; nylon 6.66; nylon 6,10; and blends and mixtures thereof. Nylon6,6 is commercially available, for example, under the trade name DARTEK®from Exopack Performance Films Inc., Whitby, Canada. Nylon 6 iscommercially available, for example, under the trade name Nylon 4.12from DuPont. Nylon 6.66 is commercially available under the trade names“ULTRAMID® C4” and “ULTRAMID® C35” from BASF, or under the trade name“UBE 5033FXD27” from Ube Industries Ltd. Heat stabilizer-modifiedversion of the above nylons, and blends and mixtures of the nylons, mayalso be used.

The laminates of this invention comply with the requirements ofparagraph h(1) of 21 CFR §177.1630 dated Apr. 1, 2003. This paragraphrequires that the food contact surface, when exposed to distilled waterat 250° F. (121° C.) for 2 hours, yields chloroform-soluble extractivesnot to exceed 0.02 mg/in² (0.0031 mg/cm²) of food contact surfaceexposed to the solvent; and that the food contact surface, when exposedto n-heptane at 150° F. (66° C.) for 2 hours, yields chloroform-solubleextractives not to exceed 0.02 mg/in² (0.0031 mg/cm²) of food contactsurface exposed to the solvent.

Capping Web

Suitable capping webs comprise a heat-sealable polymer layer, eitheralone or on the surface of a substrate layer, and in use theheat-sealable layer is eventually heat-sealed to the laminate on itsthermoformable copolyester surface. The substrate layer of the cappingweb (if present) is typically polymeric and may be monolithic, althoughother layers may be added on the side of the substrate layer oppositethe heat-sealable polymer layer. A polymeric material is the majorcomponent of the heat-sealable layer, constituting at least 50%,preferably at least 65%, preferably at least 80%, preferably at least90%, and preferably at least 95% by weight of the total weight of theheat-sealable layer. Typically, one or more tackifiers, antifog agents,etc. may make up the balance of the layer. In some cases, theheat-sealable layer consists of the polymeric material.

During heat-sealing, the polymeric material of the heat-sealable layersoftens to a sufficient extent that its viscosity becomes low enough toallow adequate wetting for it to adhere to the surface to which it isbeing bonded. The heat-seal bond is effected by heating to soften thepolymeric material of the heat-sealable layer without melting any of theother layers in either film, and applying pressure. Thus, the polymericmaterial of the heat-sealable layer should begin to soften at atemperature such that the heat-seal bond can be formed at a temperaturewhich is less than the melting temperature of the polymeric material ofthe substrate. In one embodiment, the polymeric material of theheat-sealable layer should begin to soften at a temperature such thatthe heat-seal bond can be formed at a temperature which is between about5 and 50° C. below, preferably between about 5 and 30° C. below, andpreferably at least about 10° C. below the melting temperature of thepolymer material of the substrate.

In a preferred embodiment, the heat-sealable layer comprises, andtypically consists essentially of, a copolyester resin derived from atleast one (and preferably only one) aromatic dicarboxylic acid and atleast one (and preferably only one) aliphatic dicarboxylic acid (ortheir lower alkyl (i.e. up to 14 carbon atoms) diesters) with one ormore glycol(s). Formation of the copolyester is conveniently effected inknown manner by condensation, or ester-interchange, at temperaturesgenerally up to 275° C.

Preferred aromatic dicarboxylic acids include terephthalic acid,isophthalic acid, phthalic acid, and 2,5-, 2,6- or2,7-naphthalenedicarboxylic acid, and preferably the aromaticdicarboxylic acid is terephthalic acid. Preferred aliphatic dicarboxylicacids are saturated aliphatic dicarboxylic acids of the general formulaC_(n)H_(2n) (COOH)₂ wherein n is 2 to 8, such as succinic acid, sebacicacid, adipic acid, azelaic acid, suberic acid or pimelic acid,preferably sebacic acid, adipic acid and azelaic acid, and morepreferably azelaic acid. In one embodiment, the polyester contains nomore than 90% of aromatic dicarboxylic acid (preferably TA) and at least10% of aliphatic dicarboxylic acid, the percentages being the molepercentage of the total diacid content of the polyester, provided thatthe copolyester of the heat-sealable layer is of different compositionthan the substrate layer, as discussed hereinabove with regard torelative softening temperatures. Preferably, the concentration of thearomatic dicarboxylic acid present in the heat-sealable copolyester isno more than about 80 mole %, and preferably in the range from 45 to 80mole %, more preferably 50 to 70 mole %, and particularly 55 to 65 mole% based on the dicarboxylic acid components of the copolyester. Theconcentration of the aliphatic dicarboxylic acid present in theheat-sealable copolyester is at least about 20 mole %, and preferably inthe range from 20 to 55, more preferably 30 to 50, and particularly 35to 45 mole % based on the dicarboxylic acid components of thecopolyester. In some embodiments, it is desirable to avoid the use ofdicarboxylic acid monomers that are sulphonated, i.e. dicarboxylic acidscontaining a sulphonic acid group or salt thereof (i.e. dicarboxylicacids which contain an —SO₃X moiety where X is H or an alkali metal,such as sodium sulfo-isophthalate).

Preferred glycols are aliphatic glycols, and more preferably alkyleneglycols. Thus, suitable glycol(s) include aliphatic diols such asethylene glycol, diethylene glycol, triethylene glycol, propyleneglycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol,2,2-dimethyl-1,3-propanediol, neopentyl glycol and 1,6-hexanediol.Ethylene glycol or 1,4-butanediol is preferred. Thus, the glycols aresuitably low molecular weight diols (i.e. having a molecular weightbelow about 250). In some embodiments, it is desirable to avoid the useof glycols having an average molecular weight of over 250, such aspoly(alkylene oxide) glycols. The copolyester of the heat-sealable layeris thus suitably a linear copolyester. The heat-sealable layer typicallyuses but a single polyester species, rather than a blend of differentpolyesters.

Preferably, the T_(g) of the copolyester is no more than about 20° C.,preferably no more than about 10° C., preferably no more than about 0°C., and preferably no more than about −20° C. In one embodiment, themelting point T_(m) of the copolyester is preferably no more than about160° C., preferably no more than about 150° C., more preferably no morethan about 140° C., and preferably no more than about 130° C.

Particularly preferred examples of such copolyesters are (i)copolyesters of azelaic acid and terephthalic acid with an aliphaticglycol, preferably ethylene glycol; (ii) copolyesters of adipic acid andterephthalic acid with an aliphatic glycol, preferably ethylene glycol;and (iii) copolyesters of sebacic acid and terephthalic acid with analiphatic glycol, preferably butylene glycol. Preferred polymers includea copolyester of sebacic acid/terephthalic acid/butylene glycol(preferably having the components in the relative molar ratios of45-55/55-45/100, more preferably 50/50/100) having a glass transitionpoint (T_(g)) of −40° C. and a melting point (T_(m)) of 117° C.), and acopolyester of azelaic acid/terephthalic acid/ethylene glycol(preferably having the components in the relative molar ratios of40-50/60-50/100, more preferably 45/55/100) having a T_(g) of −15° C.and a T_(m) of 150° C. The heat-sealable layer typically has a thicknessin a range from 0.5 mil to 2.0 mil (13 to 51 μm), depending on thedesired end structure.

The substrate layer may consist of any of a wide variety of materials,including but not limited to biaxially oriented polyethyleneterephthalate films, PET scrim, and Nylon films. One exemplarycoextruded film suitable for use as a capping web comprises a clear,crystalline PET layer and an amorphous copolyester heat seal layer. Afilm of this type is available from DuPont Teijin Films under the tradename MYLAR® 851. The substrate layer typically has a thickness in arange from 0.5 mil to 2.0 mil (13 to 51 μm). The substrate layer may beunoriented, but preferably it is biaxially oriented.

Optionally, the capping web is surface printed or reverse printed (i.e.printed on a face that will be internal in the final film) to providegraphics, product information and the like. Printing is advantageouslyapplied to the capping web, which is not subjected to thermoforming, sothat the graphics are not distorted. Specific examples of capping websincluding printed features include, from outermost layer to innermostlayer: PVDC Coating/nylon/ink/adhesive/PET/heat-seal layer; Nylon/PVDCCoating/ink/adhesive/PET/heat-seal layer; or Nylon/ink/adhesive/PVDCCoating/PET/heat-seal layer. In these examples, the ink layer representsprinted graphics that are applied to the second film prior to adhesivelamination, which may be effected with any of the laminating adhesivesdescribed earlier herein. For example, base films comprising orientedPET with an amorphous copolyester heat-seal layer can be adhesivelylaminated to a second film comprising a nylon layer and a PVDC barriercoating. The PVDC coating may also face the PET film.

Film Formation Processes

Films useful preparing the laminates or the capping webs may be made byvirtually any method of film forming known to those skilled in this art.They may for example be cast, extruded, co-extruded, laminated and thelike, including orientation (either uniaxially or biaxially) by variousmethodologies (e.g., blown film, mechanical stretching or the like).Various additives known to one skilled in the art can be present in therespective film layers including the presence of tie layers and thelike. Additives include antioxidants and thermal stabilizers,ultraviolet (UV) light stabilizers, pigments and dyes, fillers,delustrants, anti-slip agents, plasticizers, anti-block agents, otherprocessing aids, and the like.

Film manufacture can be carried out according to any known methods. Itis possible, for example, to manufacture a primary film by extruding thecompositions using so-called “blown film” or “flat die” methods. A blownfilm is made by extruding a polymeric composition through an annular dieand expanding the resulting tubular film with an air current to providea blown film. Cast flat films are made by extruding a compositionthrough a flat die. The film leaving the die can be cooled by at leastone roll containing internally circulating fluid (a chill roll) or by awater bath to provide a cast film.

A monolayer or multilayer film may be hot-blown from an extrusion die ata relatively high blow-up ratio. Suitable thermoplastic polyesters arepreferably crystalline and of relatively high molecular weight tomaintain film integrity during the blow-up procedure. The polyester canbe stretched sufficiently during the hot blowing process to providebalanced orientation in both the longitudinal (machine) and transversedirections.

Alternatively, thermoplastic crystalline materials may be stretchoriented to obtain a biaxially oriented film. For example, a tubing isextruded and then cooled and reheated, and then stretched by, forexample, a blown bubble process. This process is well known in the art.In the case of stretch-oriented materials, the tubing is being stretchedand oriented at a relatively low temperature in comparison with the hotblown process disclosed above.

Thermoformed Laminate Package Sealed with Capping Web

The film laminate can be used in a myriad of applications such as toform packages using existing form, fill and seal (FFS) machinesavailable from a number of manufacturers (e.g., Repak, Tiromat ULMA, andMulti-Vac). In a typical process, the laminate is thermoformed into theshape of a receptacle having a pocket with a flange and an opening, withthe copolyester film layer forming an inner surface of the pocket and amating surface of the flange. The heat-sealable layer of the capping webis sealed under vacuum to the flange to hermetically seal the package.

The packages may typically contain foodstuffs such as meat, poultry,seafood, non-meat products. Examples of foodstuffs that may be packagedin packages of this invention include processed meats such as sausages,hot dogs and the like. The foodstuffs also include value-added,seasoned, marinated and/or precooked meat products or prepared meals.The foodstuffs may also be whole-muscle and/or bone-in meat and poultryportions such as, for example but not limitation, pork loin, turkey orchicken breasts and the like. Poultry also includes ready-to-cook wholebirds. Packages may also be used to enclose fresh meat, poultry andseafood in modified atmospheric packaging applications or vacuumpackaging applications. The packages are dual-ovenable, and can bedesigned to self-vent during cooking as the heat-seal layer softens.

A preferred package of this invention consists essentially of (a) athermoformable laminate disclosed herein and (b) a capping web, in whichthe capping web is heat sealed to the laminate after it has beenthermoformed. Typically, the thermoformable laminate can be used to formpockets and then the pockets are filled with contents (for example,poultry) in an in-line packaging machine. The pockets can then be closedby heat sealing the margins of the laminate to a capping web inhorizontal form, fill and seal applications such that the laminate andthe capping web are hermetically sealed to each other.

Vent areas can be made anywhere on the package simply by providing aslit in the package face at the time of heating.

Packages made as described above are dual-ovenable, and may also beself-venting at typical cooking temperatures, depending on the choice oflaminate and capping web. Self venting is a desirable safety feature tominimize the occurrence of steam blast on opening the package. In aconventional oven, once the sealant reaches its softening point andenough pressure is generated internally, the seal will rupture and ventthe package. This venting also promotes air exchange for browning themeat if so desired. In another example, the sealant thickness and sealtemperature can be designed to provide a vent (via rupture) once thefoodstuff reaches 170° F. (76.7° C.). This may be useful when heating afoodstuff in a microwave, where the self-venting feature serves as atemperature indicator to show that the food is heated to the propertemperature and is ready for consumption.

One exemplary heat-seal polymer suitable for making seals that vent onheating is an amorphous copolyester. A coextruded film suitable forpreparation of forming webs or capping webs that can be used inself-venting packages comprises a clear PET layer and an amorphous heatseal layer. Films of this type (having various thicknesses of clear PETand heat seal layer) are available from DuPont Teijin Films under thetrade name MYLAR® OL.

In some embodiments, the package may vent when the temperature of thepackage reaches 150® F. (65.6® C.) to 450® F. (232.2® C.). In someembodiments, the package vents only at a temperature above 210® F.(98.9® C.), or only at a temperature above 250® F. (121.1® C.).

Packages according to the invention may incorporate other features suchas perforations, tear zones and the like that facilitate opening thepackage. Polyamide and PET (heat sealable) combinations provideexcellent directional tear properties in that a pre-notched package canbe torn open in a straight line in either the machine direction ortransverse direction. These tear properties provide greater flexibilityin package configuration and design. Thus, the opening area to accessthe foodstuff after heating is not limited to a particular part (e.g.the top) of the package. This can allow for locating a notch at the sideof the package in the desired area for opening.

EXAMPLES

The following test methods may be used to determine certain propertiesof the laminates of this invention, and packages made from them.

Heat-seal strength of the composite film to the capping web is measuredas follows. The capping web is sealed to the surface of thethermoformable copolyester layer of the thermoformable laminate using aSentinel sealer at a temperature of 160° C. for 1.0 second under apressure of 80 psi (0.55 N/mm²). Strips (25 mm wide) of the sealedcapping web and thermoformable laminate are cut out at 90° to the seal,and the load required to pull the seal apart measured using an Instronoperating at a crosshead speed of 0.25 m/minute. The procedure isgenerally repeated 5 times, and a mean value of 5 results calculated.

Heat-seal strength of the composite film to itself to itself is measuredby positioning together and heating the heat-sealable layers of twosamples of the film at 160° C. for 0.5 second under a pressure of 80 psi(0.55 N/mm²). The sealed film is cooled to room temperature, and thesealed composite cut into 25 mm wide strips. The heat-seal strength isdetermined by measuring the force required under linear tension per unitwidth of seal to peel the layers of the film apart at a constant speedof 0.25 m/minute.

Shrinkage is measured by placing the sample in an oven at a temperatureof 190° C. for 5 minutes and determining the average % shrinkage in boththe machine and transverse directions, based on five film samples.

Glass transition temperature is measured by Differential ScanningCalorimetry (DSC). A 10 mg polymer specimen taken from the film is driedfor 12 hours under vacuum at 80° C. The dried specimen is heated at 290°C. for 2 minutes and then quenched onto a cold block. The quenchedspecimen is heated from 0° C. to 290° C. at a rate of 20° C./minuteusing a Perkin-Elmer DSC7B Differential Scanning Calorimeter. Thecalorimeter is calibrated at a heating rate of 20° C./minute, so coolingtemperatures are corrected by adding 3.9° C. to the computer-generatedresults.

The crystallinity percentage is measured using a Perkin Elmer DSC7BDifferential Scanning Calorimeter. A 5 mg sample taken from the film isheated from 0 to 300° C. at 80° C./minute, and the percent crystallinityis calculated by methods well known in the art.

Laminate Preparation and Thermoforming

A typical process for forming a laminate according to the invention isas follows. The thermoformable copolyester film and the structural filmare corona-treated on the surfaces to be joined. A 1:1 mixture by volumeof MOR-FREE™ 225 (polyester)/MOR-FREE™ C-33 (isocyanate) is applied by agravure roll at room temperature at a rate of 2.0-2.5g/m². The laminatedfilm goes through a heated nip roll at 52-56° C. at a speed of about 500ft/min, and the film is then rolled up and stands at room temperaturefor 48 h for the initial curing to take place. The film rolls are thentransferred to heated room which is at 40-45° C. and kept for 5 days atwhich time the curing is complete. At this time the lamination bondstrengths typically range from 1000 g/25 mm to destructive. Examples 1-4describe exemplary laminates according to the invention.

A typical process of thermoforming the laminate according to theinvention is as follows. The thermoformable laminate is heated andformed using a Multivac packaging machine such as a Multivac 530 whichis equipped with a pear-shaped mold where the deepest point is about 4inches. The forming temperature is set in a range from 130° C. to 215°C., and the forming time and vacuum are typically set at 2.0 seconds and10 mbar, respectively.

EXAMPLE 1

A 2-mil biaxially oriented polyester film was laminated with 4-milDARTEK® H917 heat-stabilized nylon 6,6 film (Exopack Performance FilmsInc., Whitby, Canada) using a solventless adhesive consisting ofMOR-FREE™ 225 and MOR-FREE™ C-33 mixed at a 1:1 ratio at roomtemperature. Both films were corona treated prior to lamination. Thediacid content of the polyester was 93/7 wt/wt terephthalic acid/azelaicacid, and the diol content was 95.7/4.3 wt/wt ethylene glycol/diethyleneglycol. The amount of adhesive applied was 2.5 g/m². The adhesive wasapplied to the corona treated H917 surface and the films were laminatedtogether at 500 ft/minute. The temperature of the nip roll was 54° C.The composite film was kept at ambient temperature for 2 days and thenmoved to the heated room which was at 43° C. for 5 days to complete thecuring process.

EXAMPLE 2

A corona-treated 2-mil polyester film having the same polyestercomposition as in Example 1 but with a 1.5 μm PVdC coating on the sideopposite the corona treatment is laminated to a corona-treated 4-milDARTEK® H917 heat-stabilized nylon 6,6 film. The laminating adhesive isa solventless adhesive consisting of MOR-FREE™ 225 and MOR-FREE™ C33mixed at a 1:1 ratio at room temperature. The adhesive loading is 2.5g/m². The adhesive is applied to the corona treated H917 surface and thePET film surface. The temperature of the nip roll is 54° C. Thecomposite laminate is kept at ambient temperature for 2 days and thenplaced in a heated room at 43° C. for 5 days to complete the curingprocess.

EXAMPLE 3

A 2-mil polyester film having the same composition as the polyester inExample 1 was laminated to 2-mil DARTEK® H917 heat-stabilized nylon 6,6film. A process similar to that of Example 1 was followed.

EXAMPLE 4

A corona-treated 2-mil PVdC coated polyester film as described inExample 2 is laminated to 2-mil DARTEK® H917 heat-stabilized nylon 6,6film, using a process similar to that of Example 2.

Capping Webs

Examples 5-7 describe exemplary capping webs suitable for use with thethermoformable copolyester laminates of this invention.

EXAMPLE 5

A 1-mil biaxially oriented PET film with an amorphous copolyestersealant layer (MYLAR® OL13, DuPont Teijin Films, Richmond, Va.) waslaminated to 1-mil DARTEK® H917 heat-stabilized nylon 6,6 film. Asimilar process to Example 1 was followed. The non sealant side of thepolyester film was corona treated and in contact with the adhesive whenthe laminate was formed.

EXAMPLE 6

A 1-mil polyester film with an amorphous copolyester sealant layer onone side and a PVdC barrier layer on the other (Mylar® OB13, DuPontTeijin Films, Richmond, Va.) was laminated to 1-mil DARTEK® H917heat-stabilized nylon 6,6 film, with the barrier layer adhered to thenylon. A process similar to that of Example 2 was followed.

EXAMPLE 7

A 1.2-mil polyester film having an amorphous copolyester heat-seal layeron one side (MYLAR® OL22, DuPont Teijin Films, Richmond, Va.) waslaminated to 1-mil DARTEK® H917 heat-stabilized nylon 6,6 film. Aprocess similar to that of Example 1 was followed. The non sealant sideof the MYLAR® OL22 was corona treated and in contact with the adhesivewhen the laminate was formed.

EXAMPLE 8

A thermoformable copolyester laminate made as in Example 1 was used as aformable web, and a laminate made as in Example 5 was used as thecapping web, as follows. A Multivac R530 packaging machine was equippedwith a pear-shaped mold where the deepest point was about 4 inches, andthe form temperature was set to 160° C. The thermoformable laminate washeated and formed at the same location, using a forming time of about2.0 seconds and a vacuum of about 10 mbar.

The cavity was then filled with product, a marinated sirloin roasthaving a weight of about 1 lb 12 oz, leaving about ½-1 inch head space,and the package was sealed with the capping web at 180° C. with a dwellor sealing time of about 1.0 second. A total heating seal plate wasused, but in typical practice a perimeter sealing plate may be used.

Two sirloin roasts each having a weight of about 1 lb 12 oz weremarinated with seasonings and vacuum packaged as described above. Themeats were frozen solid, and one was removed from its package just priorto cooking. The packaged and unpackaged roasts were cooked side-by-sidewithout thawing in a conventional oven set to 350° F. After 25 minutesin the oven, the package spontaneously opened by venting through a tinyvent and continued to cook under partial steam in a bubble with a raisedtop web. Elapsed times to reach 160° F. internal meat temperature were75 and 83 minutes for the packaged and unpackaged roasts, respectively.The roast that had been cooked in the package was distinctly more juicyand moist than the unpackaged roast.

EXAMPLE 9

A turkey roast having a weight of about 9 lbs was marinated withseasonings and vacuum packaged as follows. A Multivac R530 packagingmachine was equipped with a pear shaped mold where the deepest point wasabout 4 inches, and the form temperature was set to 160° C. The laminatefilm of Example 1 was heated and formed at the same location, using aforming time of about 2.0 seconds and a vacuum of about 10 mbar.

The cavity was then filled with product, leaving about ½-1 inch headspace, and the package was sealed with the capping web made in Example 7at 180° C. with a dwell or sealing time of about 2.0 second. A totalheating seal plate was used, but in typical practice a perimeter sealingplate may be used.

The packaged meat was placed in an Alkar™ oven set to 190° F. for sixhours. The package was removed and frozen in a freezer. No venting orrupture of the seal occurred. The package was taken from the freezer andplaced in a conventional oven set to 250° F. for reheat. After 30minutes in the oven, the package spontaneously opened by venting througha tiny vent and continued to be heated under partial steam in a bubblewith a raised top web for a total of 90 minutes reheat time.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimswithout departing from the invention.

1. A laminate comprising the following substantially coextensive layersin the following order (a) a non-sealable, self-supporting,thermoformable copolyester film layer having a first surface and asecond surface, said second surface constituting an outermost, exposedsurface of the laminate, wherein the copolyester in the thermoformablecopolyester film layer comprises from 90 mol % to 97 mol % of at leastone aromatic dicarboxylic acid and correspondingly from 10 mol % to 3mol % of at least one aliphatic dicarboxylic acid; (b) a laminatingadhesive layer on the first surface of the thermoformable copolyesterfilm layer; and (c) a self-supporting, thermoformable structural filmlayer having a first surface and a second surface, said first surfacecontacting the laminating adhesive layer; wherein polyethyleneterephthalate constitutes at least 80% by weight of the self-supportingthermoformable copolyester film layer; the thermoformabie structuralfilm layer comprises a polymer selected from the group consisting ofpolyamides, polypropylene, polyethylene, polyethylene terephthalate,ionomers, ethylene acrylic acid copolymers, ethylene vinyl acetatecopolymers, polystyrene, ethylene vinyl alcohol copolymers andpolyvinylidene chloride; the thermoformabie copolyester film layer, thestructural film layer and the laminate each shrink less than 5% inlength and width upon exposure to bulling water for five seconds; andthe laminate is thermoformable and its chloroform-soluble extractivesmeet the requirements of paragraph h(1) of 21 CFR §177.1630 as definedherein,
 2. The laminate of claim 1, wherein the structural film layercomprises a polyamide.
 3. The laminate of claim 1, wherein thestructural film layer comprises nylon 6,6.
 4. The laminate of claim 1,further comprising a barrier layer substantially coextensive with thecopolyester film layer and the structural film layer, on a side of thestructural film layer opposite the copolyester film layer, wherein thebarrier layer comprises a polymer selected from the group consisting oforiented polyethylene terephthalate, polyvinylidene chloride, ethylenevinyl alcohol copolymers and polyamides.
 5. The laminate of claim 4,consisting of the copolyester film layer, the laminating adhesive layer,the structural film layer, and the barrier layer.
 6. The laminate ofclaim 4, wherein the barrier layer comprises polyvinylidene chloride. 7.The laminate of claim 4, wherein the barrier layer is on the secondsurface of the structural film layer.
 8. The laminate of claim 1,consisting of the copolyester film layer, the laminating adhesive layerand the structural film layer.
 9. The laminate of claim 1, wherein thelaminate is in the shape of a thermoformed receptacle haying a pocketwith a flange and an opening, the second surface of the copolyester filmlayer forming an inner surface of the pocket and a mating surface of theflange.
 10. A package comprising the laminate of claim 9 and aheat-sealable film, wherein the opening of the receptacle is closed bythe heat-sealable film, said heat-sealable film comprising aself-supporting polymeric film having on a surface thereof asubstantially coextensive heat-seal layer; wherein the heat-seal layerof the heat-sealable film is in contact with and bonded to the matingsurface of the flange of the thermoformed receptacle; and whereinchloroform-soluble extractives of the heat-sealable film meet therequirements of paragraph h(1) of 21 CFR §177.1630 as defined herein.11. The package of claim 10, wherein the heat-seal layer comprises anamorphous copolyester.
 12. The package of claim 10, wherein the laminateis capable of forming a heat-seal bond to the capping web with a bondstrength of at least 250 g/25 mm and at most 2500 g/25 mm.
 13. Thepackage of claim 10, further comprising a contained foodstuff.
 14. Thepackage of claim 10, wherein the heat-seal layer ruptures and vents thepackage at a temperature in a range from 150° F. (65.6° C.) to 450° F.(232.2° C.).
 15. The package of claim 14, wherein the heat-seal layerruptures and the package vents only at a temperature above 210° F.(98.9° C.).
 16. The package of claim 14, wherein the heat-seal layerruptures and the package vents only at a temperature above 250° F.(121.1° C.).
 17. The package of claim 13, wherein the foodstuff is meat,poultry, or seafood.
 18. A method of packaging a foodstuff, comprisingplacing the foodstuff in the pocket of the laminate of claim 9 andsubsequently sealing the opening by heat-sealing a film to the flange.19. The package of claim 14, wherein the foodstuff is meat, poultry, orseafood.